A functionalized ionic liquid containing phosphine-ligated palladium complex for the Sonogashira reactions under aerobic and CuI-free conditions

Article abstract of DOI:10.1016/j.catcom.2011.10.031

A Pd-complex functionalized ionic liquid (IL), di-(1-butyl-2- diphenylphosphino-3-methylimidazolium)-dichloropalladium(II) hexafluorophosphate (2), was synthesized and used as the highly efficient and recyclable catalyst for the Sonogashira reactions of aryl iodides and aryl bromides with several terminal acetylenes, under aerobic and CuI-free conditions. The activity loss, Pd black precipitation, and Pd leaching were not observed even after 7 runs in 2-catalyzed Sonogashira reactions using the room temperature IL of [Bmim]PF ₆ as the reaction medium.

Full text of DOI:10.1016/j.catcom.2011.10.031

Catalysis Communications 17 (2012) 160–163
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A functionalized ionic liquid containing phosphine-ligated palladium complex for the
Sonogashira reactions under aerobic and CuI-free conditions
a,1
b,2
b,2
a,
a,
Jing Zhang , Marijana Đaković , Zora Popović , Haihong Wu ⁎, Ye Liu ⁎
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Chemistry Department of East China Normal University, Shanghai 200062, PR China
Chemistry Department, University of Zagreb, HR-10000 Zagreb, Croatia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 August 2011
Received in revised form 17 October 2011
Accepted 28 October 2011
Available online 4 November 2011
A Pd-complex functionalized ionic liquid (IL), di-(1-butyl-2-diphenylphosphino-3-methylimidazolium)-
dichloropalladium(II) hexafluorophosphate (2), was synthesized and used as the highly efficient and recycla-
ble catalyst for the Sonogashira reactions of aryl iodides and aryl bromides with several terminal acetylenes,
under aerobic and CuI-free conditions. The activity loss, Pd black precipitation, and Pd leaching were not
observed even after 7 runs in 2-catalyzed Sonogashira reactions using the room temperature IL of [Bmim]PF
as the reaction medium.
6
Keywords:
©
2011 Elsevier B.V. All rights reserved.
Sonogashira reaction
Functionalized ionic liquids
Palladium complex
1
. Introduction
The Sonogashira reaction is a powerful tool for the formation of
for more efficient and clean Sonogashira reactions, including copper-
and ligand-free reactions [9,10], new palladium catalyst systems [11],
as well as significant changes in the reaction conditions and solvents
[12–17]. Non-volatile ionic liquids (ILs) have been recognized as
promising alternative solvents, which also can immobilize homogenous
palladium catalysts and then prevent their leaching and deactivation
[15–17]. On the other hand, ILs can be functionalized flexibly by varying
the cation and/or anion that constitute IL structure with specific func-
tionalities. Thus, it is possible to develop abundant functionalized ILs
(FILs) dually possessing the characters of the incorporated functionali-
ties as well as that of the ILs. As a part of our continued activities in the
functionalization of ILs for homogeneous catalysis [18–22], herein, for
the first time we report the synthesis of a phosphine-ligated palladium
complex FIL, di-(1-butyl-2-diphenylphosphino-3-methylimidazolium)-
dichloridopalladium(II) hexafluorophosphate (2), which was applied
as the efficient and recyclable catalyst for the clean Sonogashira reac-
tions under aerobic and CuI-free conditions (Scheme 1). Due to high
polarity of 2 and its compatibility with the room temperature IL of 1-n-
2
Csp \Csp bond, and the most important method for the synthesis of
internal acetylenes, which finds its applications in the preparation of
natural products, pharmaceuticals, biologically active molecules, liq-
uid crystalline materials and conducting polymers, or molecular elec-
tronics in general [1–3]. In the conventional method, the Sonogashira
coupling is performed in the presence of base, copper(I) iodide as co-
catalyst, and palladium catalyst such as PdCl
PPh , PdCl /PPh , or Pd(PPh in organic solvents under inert atmo-
2 3 2 2
(PPh ) , [Pd(allyl)Cl] /
3
2
3
3 4
)
sphere [4–6]. The inert atmosphere is required for the Sonogashira
reaction due to the following reasons: 1) most of phosphine-containing
palladium catalysts are moisture- and air-sensitive; 2) the in situ formed
copper(I) acetylides derived from copper(I) iodide, undergo homocou-
pling upon exposure to air, yielding the side-products (Glaser coupling);
and 3) the copper(I) acetylides are featured with the extremely high
reactivity and the explosive nature in open air [1–3,7,8].
From environmental and economic points of view, the develop-
ment of more efficient, recoverable and recyclable, but moisture-
and air-insensitive palladium catalysts, which allow the manipulation
of the Sonogashira reaction with low catalyst loadings under copper-
free conditions and without the need of exclusion of air and moisture,
is highly demanded. Recent researches have led to many alternatives
6
butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF ), the lat-
ter was selected as the solvent to assure the homogeneous catalysis. The
performed Sonogashira reactions are expected with advantages of no
need of inert atmosphere, CuI-free condition, as well as the available
6
recyclability of 2 in [Bmim]PF .
2
. Results and discussion
⁎
Corresponding authors. Tel.: +86 21 62232078; fax: +86 21 62233424.
E-mail addresses: hhwu@chem.ecnu.edu.cn (H. Wu), yliu@chem.ecnu.edu.cn
2.1. Synthesis and characterization of 2
(
Y. Liu).
1
When PdCl
2 3 2
(CH CN) was treated with ligand 1 (1, 1-butyl-2-
Tel.: +86 21 62232078; fax: +86 21 62233424.
2
Fax: +385 1 4606341.
diphenylphosphino-3-methylimidazolium hexafluorophosphate [23])
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.031

J. Zhang et al. / Catalysis Communications 17 (2012) 160–163
161
6
Scheme 1. The Sonogashira reactions of aryl halides with terminal acetylenes catalyzed by 2 in [Bmim]PF .
in ethanol, 2 was obtained as a yellow solid in good yield (75 wt.%)
which was fully characterized by 1H NMR, P NMR, TG/DTA, and
elemental analysis. It is moisture- and air-stable both in solid state
and in organic solvent at room temperature. The crystals suitable for
single-crystal X-ray analysis were obtained by re-crystallization from
acetone/benzene.
against oxidative degradation, which facilitates the manipulation in
open air.
31
2.2. Catalytic performance of 2 for the Sonogashira reaction
The molecular structure of 2, depicted in Fig. 1, is very similar to that
The coupling of phenylacetylene with iodobenzene was first stud-
ied as a model reaction under CuI-free conditions. A typical reaction
was carried out by mixing 1 mmol iodobenzene, 1.5 mmol phenylace-
of the neutral palladium complex of trans-PdCl
2
(PPh
3
)
3
[24]. 2 is com-
−
posed of the Pd complex cations and PF
6
anions. The Pd complex cation
possesses square-planar geometry. The Pd center, lying at the center of
inversion, is coordinated by two chloride ions and two imidazolium-
tailed phosphine ligands. The bond angles of P1\Pd1\P1A and
Cl1\Pd1\Cl1A are 180°. The same bond distances of 2.3203(7)Å for
Pd\P and 2.2967(7)Å for Pd\Cl are observed. The bond distances of
Pd\P and Pd\Cl in 2 (Table 1) are slightly shorter than those observed
tylene, 2 mmol base, and 0.02 mmol 2 in 2 mL of [Bmim]PF
appointed conditions. The selection of [Bmim]PF as the solvent is
made on the comparison of its same ionophilicity and the structural
similarity with 2. Through screening the bases, Et N was found to
6
under the
6
3
be the best choice. Under the optimal condition (2 h, 80 °C), the com-
plete conversion of iodobenzene into the cross-coupling product was
obtained.
The purpose of incorporating the palladium complex into the IL
structure is to integrate the advantageous characters of the palladium
complex as the catalyst and the IL like recyclability. To demonstrate
that issue, the recovery and recycling experiments were conducted
in trans-PdCl
2
(PPh
3
)
3
(Pd\P, 2.3721(10)Å; Pd\Cl, 2.3111(13)Å) [24].
, ligand 1 is a relatively weaker electron-
In comparison with PPh
3
donor featured with moisture- and air-insensitive character, due to
the intensive delocalization of the lone-pair electrons in the P atom
to the adjacent positive imidazolium ring. Consequently, 2 is also a
moisture- and air-insensitive complex endowed with strong tolerance
6
over 2 in [Bmim]PF for the coupling of phenylacetylene with iodo-
benzene. Upon completion of each run, the reaction slurry was
extracted with diethyl ether to separate the substrate and the prod-
ucts out of the IL phase. The residues were reused without further
treatment if not specified. As shown in Table 2, the activity of 2
decreased gradually although the precipitation of Pd black was not
observed during the recycling. After six runs, the yield of 1,2-
diphenylethyne decreased from 100% to 66%. However, when the res-
idues were washed with water to remove the cumulative ammonium
salts, the recovered IL phase exhibited the activity as well as the fresh
one. These results indicated that the activity loss was due to the cap-
sulation of 2 by the formed slurry of ammonium salts, thus leading to
the inefficient accessibility of the catalytic site to the substrate and
then the limited transformation. In this method, the hydrophobicity
6
of [Bmim]PF and 2 facilitated the removal of the formed ammonium
salts by water from the IL system, leading to the convenient refresh-
ment of the activity of 2. On the other hand, after the seven-run recy-
cling uses, the total loss of 2 in term of Pd amount in the combined
organic and water phase was below the ICP detection limit (b0.1 μg/g
on the basis of the ICP analysis). This revealed that 2 was tightly locked
in the structurally similar [Bmim]PF
the process of ether extraction or water washing. In contrast, when
CH CN was used as the solvent, the activity of 2 lost rapidly due to the
6
medium without leaching during
3
Table 1
2 3 2
The selected bond distances (Å) and bond angles (o) for 2 and trans-PdCl (PPh ) .
2
Pd1–Cl
2.297(7)
92.62(3)
180.0
2.311(1)
92.38(4)
180.0
Pd1–P
2.320(7)
87.38(3)
180.0
2.372 (1)
87.62(4)
180.0
Cl1–Pd1–P1
Cl1–Pd1–Cl1A
Pd1–Cl
Cl1–Pd–P1
Cl1–Pd1–Cl1A
Cl1A–Pd1–P1
P1–Pd1–P1A
Pd1–P
Cl1A–Pd–P1
P1–Pd1–P1A
trans-PdCl
2
(PPh
3
2
) [24]
Fig. 1. Molecular structure of 2 (the solvent molecule of benzene has been omitted for
clarity).

162
J. Zhang et al. / Catalysis Communications 17 (2012) 160–163
Table 2
Table 3
The recycling uses of 2 as catalyst for the Sonogashira reaction of phenylacetylene with
iodobenzene .
The generality of 2 as catalyst for the Sonogashira reactions of terminal acetylenes with
aryl halides in [Bmim]PF .
6
a
a
Run
Yield (%)
Bmim]PF b
Entry
1
Aryl halide
Terminal acetylenes
Time (h)
3
Yield (%)c
38
[
6
CH
3
CNc
1
2
3
4
5
6
100
98
94
79
78
66
92
98
89
78
70
48
2
3
82
3
4
3
3
71
36
Undone
Undone
d
7
a
2
3
2 mol% (0.02 mmol), iodobenzene 1 mmol, phenylacetylene 1.5 mmol, Et N
2
mmol, reaction time 2 h, temperature 80 °C, time 2 h, solvent 2 mL.
b
The total loss of Pd was undetectable by ICP analysis.
The concentration of Pd in the combined organic phase after 5 runs was 7.2 μg/g.
c
5
3
63
d
The residues after the sixth run were washed with water to remove the cumulative
ammonium salts.
6
7
3
3
57
36
serious leaching of the Pd catalyst into the organic phase. After 5 runs,
it was found that the concentration of Pd in the combined organic
solution was 7.2 μg/g (ICP analysis), indicating 34% loss of Pd into the
organic phase.
The generality of 2 as catalyst in [Bmim]PF for the Sonogashira
6
reaction was shown in Table 3, in which a wide range of the sub-
strates with different electronic and steric effects were investigated.
8
9
3
93
3
2
97
98
1
0
1
6
The results showed that the catalytic system of 2-[Bmim]PF was gen-
erally efficient to the coupling of aryl iodides with phenylacetylene.
The electronic and steric properties of the substituents also showed
the obvious influences on the reaction rate. The aryl iodides with
electron-donating substituents, such as methyl and methoxyl, reacted
with phenylacetylene to generate the corresponding coupling prod-
ucts in good to moderate yields (Entries 1–6). The substituent located
at meta- or para-position gave rise to the reaction in comparison to
that at ortho-position due to the steric effect. The activated aryl iodides
1
2
100
71
81
83
24
12b
13b
2
2
1
1
4b
5b
2
3 2
with electron-withdrawing substituent such as \CF or \NO , coupled
2.5
with phenylacetylene in excellent yields when the substituent was
located at meta- or para-position (Entries 8–11). As for the aryl bro-
mides with relatively low reactivity, the activated substrates with
electron-deficient character, such as 4-bromobenzaldehyde and 4-
bromonitrobenzene, coupled with phenylacetylene in better yields
than non-activated bromobenzene as the reaction temperature was
raised to 120 °C (Entries 13, 14 vs 12). The deactivated substrates (1-
bromo-3,5-dimethylbenzene and 1-bromo-4-methoxybenzene) were
converted to the corresponding coupling products in relatively low
yields even harsh reaction conditions were applied (Entries 15 and
1
1
1
1
6b
2
6
6
2
25
3
7b
8b
5
9
0
99
1
6). All attempts by using chlorobenzene or 4-chloronitrobenzene to
perform the Sonogashira coupling under the applied conditions have
failed so far (Entries 17 and 18). When other terminal acetylenes
were examined to couple with iodobenzene, the excellent yields of
the products were obtained without obvious electronic and steric dis-
crimination as shown in Entries 19–22.
2
2
2
93
1
2
2
90
96
22
a
2
3
2 mol% (0.02 mmol), aryl halide 1 mmol, terminal acetylene 1.5 mmol, Et N
3
. Conclusions
2
mmol, temperature 80 °C, [Bmim]PF
6
2 mL.
b
Temperature 120 °C.
c
The novel functionalized IL (2) containing the phosphine-ligated
GC and GC-mass analysis.
Pd(II) complex, featured with ionophilicity, hydrophobicity, and
moisture- and air-insensitivity, was proved to be a highly efficient,
recoverable and recyclable catalyst for the Sonogashira reaction of
phenylacetylene with iodobenzene under aerobic and CuI-free condi-
including sterically hindered and electron-rich aryl iodides and aryl
bromides, and several terminal acetylenes.
tions when the structurally similar room temperature IL of [Bmim]PF
was used as the solvent. Due to their compatibility, 2 was tightly
locked in [Bmim]PF , thus avoiding the Pd leaching and facilitating
the reuses of 2. 2-[Bmim]PF IL systems could successfully catalyze
the Sonogashira reactions available for a wide range of substrates
6
Acknowledgments
6
This work was financially supported by Science and Technology
Commission of Shanghai Municipality (No. 09JC1404800), the
National Natural Science Foundation of China (Nos. 20973063 and
6

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